In an alternate Earth, Batman kills the Joker but gets infected (to the core) by a poison gas set to be released when the Joker dies. The gas is designed in such way as to turn whoever gets infected into a "new" Joker.....thus creating the Batman Who Laughs. That is, Bruce Wayne with all the mental prowess he possesses combined with the madness of the Joker. He then proceeds to kill every hero that stands in his way (including all of the JLA) and make the world his bitch.
He is the leader of a group of alternate Batman versions and (probably) the main antagonist of the Dark Nights: Metal storyline. Designed by Greg Capullo.....

I think DST did a better job with him than the one announced by DC Collectibles the other day. Both of them should have included the 3 Robins though.... that chain looks pointless on its own.